Acid base and pH

Question

Determine the hydronium ion concentration in a solution that is 0.0001 M HCl.

Answer 1

HCl is a strong acid and it dissociates completely as follows:

a) HCl ==> H⁺ + Cl^-

b) HCl + H₂O ==> H₃O⁺ + Cl^-

Both (a) and (b) are equivalent but since your question asks about "hydronium ion" which is H3O+ I've shown both. The more conventional way is (a) and (b) is going out of favor. That being said...

hydronium ion concentration [H₃O⁺] = 0.0001 M